The Feasibility of a Cane Toad Barrier

By Darren Southwell and Reid Tingley

Preventing the spread of cane toads into Western Australia’s Pilbara could cost less than $100,000 per year.

Cane toads are one of Australia’s worst invasive species. Over the past 85 years they have spread across more than 1 million km2 of northern Australia. Along the way, the toads have had severe impacts on native biodiversity, such as goannas and quolls.

The toads seem unstoppable, but new research suggests there may be a chink in their seemingly impenetrable armour. The toad’s weakness, it seems, is its inability to retain water.

Cane toads lose water at the same rate as a sponge; they need access to water every 3–4 days or else they dehydrate and die. As the toads have penetrated further into arid regions of Australia, they have increasingly relied on artificial water sources – such as pastoral dams and tanks – to rehydrate and breed. It makes sense, then, that we might be able to stop the toad invasion in arid areas by preventing toads from accessing these water points. But where might we apply such a strategy?

It turns out that a thin “corridor” of pastoral land between Broome and Port Hedland, in Western Australia, is the perfect trap. Here the Great Sandy Desert almost reaches the coast. The cane toads will really have to squeeze through this narrow bottleneck to reach the Pilbara and continue spreading through Western Australia.

A previous study, using computer simulations, suggested that replacing about 100 pastoral dams and tanks with leak-proof tank and trough systems along this stretch of coastline just might do the trick. A number of non-government organisations and local management groups expressed interest in the waterless barrier idea, but others had reservations.

Why the doubt? Were they concerned with the data and assumptions underpinning the simulation? Or did they just misunderstand its limitations and capabilities?

To find out, we decided to ask them. To do that, we ran a workshop in Broome with local practitioners and experts in cane toad biology – the people with intimate knowledge of the landscape and of the invader. Attendees revealed that they were most concerned about the accuracy of input data going into the model, such as rainfall variability and the locations of dams, and other land uses in the corridor that might support toad populations.

In response to those concerns, we updated our previous simulation to incorporate this information. We drove the entire length of the corridor, verifying the locations of artificial and natural waterbodies on every property between Broome and Port Hedland. We also mapped a variety of other points that could potentially provide refuges for toads, such as dwellings, homesteads and roadhouses, as well as regions of irrigation and cropping. Our work enabled us to produce the most up-to-date map of permanent water and land use on pastoral land between Broome and Port Hedland.

With these updated maps, we then investigated the most cost-effective location for a waterless barrier in the corridor. This involved simulating the spread of toads through the region in the absence of waterbody management, and then testing barriers placed at 17 potential locations. An economic model estimating the upfront installation and ongoing maintenance costs of a barrier was developed to find the most cost-effective barrier location. The updated model was run with and without local knowledge of the landscape to determine if this information affected the best barrier location.

The results of the analysis suggest that the toad invasion front could be contained by excluding toads from fewer than 100 waterbodies, at a cost of approximately $4.5 million over 50 years. That’s less than $100,000 per year.

This is considerably less than the amount spent on other invasive species management programs in Australia. For example, the Australian government recently spent $19 million on feral camels in Central Australia over 4 years, and $35 million on the fox eradication program in Tasmania over 8 years.

Importantly, our research demonstrates the importance of engaging practitioners during model development. Local knowledge overlooked in the original incarnation of the model – specifically, knowledge about irrigation and dwellings – influenced the best location for a barrier.

Our new research suggests that the idea of a waterless barrier to halt the spread of cane toads is both feasible and cost-effective.* It is also a win–win situation for pastoralists and conservationists, as the installation of leak-proof tanks could improve farm productivity while simultaneously mitigating a key threatening process for biodiversity.